In the Northern Hemisphere (around the North Pole and the Arctic Ocean) the sea ice reaches its maximum extent in early spring, at the end of the long, cold winter. March is usually the month with the most sea ice.

The ice pack melts and breaks up over the summer. September is usually the month with the least sea ice (in the Northern Hemisphere).

The Northern Hemisphere ice pack seems to be shrinking, apparently as a result of global warming. The average rate of decrease in extent of the ice pack in September between 1979 and 2011 was 12.0% per decade (+ or - 2.8%).

The pink line in the images shows the average (from 1979 to 2000) edge of the ice pack for a particular month.